The present invention relates generally to lithographic printing of features for forming integrated circuit (IC) patterns on a semiconductor chip, and more particularly to a method for selecting and using combinations of illumination source characteristics and diffracting shapes on a reticle mask in order to project and print an image on a semiconductor wafer that substantially matches the shape of the desired IC patterns with minimal distortion.
Many methods have been developed to compensate for the image degradation that occurs when the resolution of optical lithography systems approaches the critical dimensions (CD""s) of desired lithographic patterns that are used to form devices and integrated circuits (IC""s) on a semiconductor chip. Critical dimension (CD) refers to the feature size and spacing between features and feature repeats (pitch) that are required by the design specifications and are critical for the proper functioning of the devices on a chip. When the CD""s of a desired IC pattern approach the resolution of a lithographic system (defined as the smallest dimensions that can be reliably printed by the system), image distortions becomes a significant problem. Today the limited resolution of lithography tools poses a key technical challenge in IC manufacture, and this difficulty will increase in the future as critical dimensions become increasingly smaller. In order to make the manufacture of future IC products feasible, lithography tools will be required to achieve adequate image fidelity when the ratio of minimum CD to resolution of the lithographic system is very low.
The resolution xcfx81 of a lithographic system can be described by the equation:                               ρ          =                                    k              ⁢                              xe2x80x83                            ⁢              λ                        NA                          ,                            [        1        ]            
where xcfx81 is the minimum feature size that can be lithographically printed, NA (numerical aperture) is a measure of the amount of light that can be collected by the lens, and xcex is the wavelength of the source light. This equation expresses the concept that the smallest feature size that can be printed is proportional to the wavelength of the light source, and that the image fidelity is improved as diffracted light is collected by the lens over a wider range of directions. Although a larger NA permits smaller features to be printed, in practice NA is limited by depth-of-focus requirements, by polarization and thin-film effects, and by difficulties in lens design. The so-called k-factor represents aspects of the lithographic process other than wavelength or numerical aperture, such as resist properties or the use of enhanced masks. Typical k-factor values in the prior art range from about 0.7 to 0.4. Because of limitations in reducing wavelength xcex or increasing numerical aperture NA, the manufacture of future IC products having very small CD""s will require reducing the k-factor, for example, to the range 0.3-0.4 or smaller, in order to improve the resolution of the lithographic processes.
The basic components of a projection lithographic system are illustrated in FIG. 1. An illumination source 110 provides radiation that illuminates a mask 120, also known as a reticle; the terms mask and reticle may be used interchangeably. The reticle 120 includes features that act to diffract the illuminating radiation through a lens 140 which projects an image onto an image plane, for example, a semiconductor wafer 150. The aggregate amount of radiation transmitted from the reticle 120 to the lens 140 may be controlled by a pupil 130. The illumination source 110 may be capable of controlling various source parameters such as direction and intensity. The wafer 150 typically includes a photoactive material (known as a resist). When the resist is exposed to the projected image, the developed features closely conform to the desired pattern of features required for the desired IC circuit and devices.
The pattern of features on the reticle 120 acts as a diffracting structure analogous to a diffraction grating which transmits radiation patterns that may interfere constructively or destructively. This pattern of constructive and destructive interference can be conveniently described in terms of a Fourier transform in space based on spacing of the features of the diffraction grating (or reticle 120). The Fourier components of diffracted energy associated with the spatial frequencies of the diffracting structure are known in the art as diffracted orders. For example, the zeroth order is associated with the DC component, but higher orders are related to the wavelength of the illuminating radiation and inversely related to the spacing (known as pitch) between repeating diffracting features. When the pitch of features is smaller, the angle of diffraction is larger, so that higher diffracted orders will be diffracted at angles larger than the numerical aperture of the lens.
A diagram can be constructed in direction space to indicate the diffracted orders that can be collected by a lithographic system that is based on repeating dimensions of a desired pattern. For example, the pattern illustrated in FIG. 4 can be represented by a unit cell as in FIG. 2. The pattern has a horizontal repeat dimension 203, and a staggered pitch indicated by the diagonal repeat dimension 205 (alternatively indicated by the vertical pitch 201). Assuming that this unit cell is repeated in a diffraction grating and illuminated by an on-axis beam, the diffracted orders can be illustrated in direction space as indicated in FIG. 3. The position of a diffracted order (points 300-326) is plotted as the projection of the beam diffracted at an angle xcex8 from the on-axis beam. The distance of a non-zero order from the center of the direction space diagram 300 (which represents the position of the zeroth order and is also the direction of the on-axis beam) is plotted as the sine of xcex8 which is the ratio of the wavelength of the illumination divided by the repeat distance. For example, the +2 order represented by the horizontal repeat distance 203 is represented by the point 301 and the xe2x88x922 order is represented by the point 310. Similarly, points 305 and 319 represent the +2 and xe2x88x922 orders based on the vertical repeat distance 201. Other orders are diffracted both horizontally and vertically, such as order 308, denoted as the {xe2x88x921, +1} order. For reference, the numerical aperture (NA) 350 of the lens is also plotted. The only orders collected by the lens are 300, 301, 310, 303, 308, 313, and 312. Note that the amplitudes of a wave front diffracted by a reticle will be dependent on both the illumination amplitude and the diffractive properties of the mask.
Off-axis illumination has been known in the art as a technique used to improve resolution. Although off-axis illumination causes asymmetry in the projected image, the asymmetry caused by the off-axis illumination can be corrected by illuminating from mirrored directions. This technique is often used in the prior art, for example, by using an annular source configuration.
The intensity contours of light projected by the lens can depart significantly in shape from those of the input mask pattern. Two dimensional (2D) patterns have multiple critical dimensions that must be met, thus exacerbating the problem of achieving image fidelity. Moreover, with all but the simplest shapes, the errors in the different critical dimensions that comprise the printed pattern are unequal, making it impossible to correct the errors with an exposure adjustment. Quite often such unequal dimensional distortions fall into the broad category of xe2x80x9cline-shorteningxe2x80x9d. For example, patterns such as in FIG. 4 (for example, an isolation level of a dynamic random access memory (DRAM) design) or as in FIG. 14 (for example, the capacitor level of a DRAM design) are prone to line-shortening. In the pattern of FIG. 4, the rectangular features, have width 401 equal to the basic dimensional unit of the cell F. The rectangular features represent regions where photoactive material (resist) should be retained after the printed pattern is developed. The vertical spacing 402 is also equal to F, and the length 405 is equal to 6.5 F. The desired horizontal spacing 408 between the tips of the rectangles is 1.5 F. However, when the k-factor is small, the contrast across the tips is small, and in order to adequately resolve the tips of the rectangles, it is necessary to print the rectangles at a length shorter than the desired 6.5 F.
In addition, at small k-factor, the low contrast of the projected images magnifies the dimensional errors that arise from random variations in the patterning process. This can cause prohibitive sensitivities to such imperfections as non-uniform substrate reflectivity, mask dimensional inaccuracy, illumination nonuniformity, defocus, stray light, and residual lens aberrations.
Many methods have been developed to reduce these problems. A summary of these prior art methods is briefly described below.
Many enhancement techniques adjust the shapes of mask features to compensate the distortions that arise at small k-factor, as discussed in L. W. Liebmann et al., xe2x80x9cOptical proximity correction: a first look at manufacturability,xe2x80x9d in SPIE Proceedings, Vol. 2322xe2x80x9414th Annual Symposium on Photomask Technology and Management, (Society of Photo-Optical Instrumentation Engineers, 1994), pages 229-238. The technique of altering the reticle mask shapes (for example, by widening the mask shapes at the tips of line features, or by lengthening the features) is referred to as biasing. For example, FIG. 4A illustrates a mask biased with hammerhead shapes 420 to compensate for line-shortening of the pattern in FIG. 4. In some cases, however, this not only fails to address the problem of poor contrast, it actually exacerbates it, i.e. biasing mask features can actually degrade contrast to the point of being counterproductive. In cases such as the pattern in FIG. 4, the contrast across the tips is poor even when the rectangles print with considerable shortening; i.e. considerable light spills between adjacent tips in the blurred image, even though shortening draws the tips apart. When the mask rectangles are biased with hammerhead shapes 420 as in FIG. 4A in order to compensate for line-shortening, the contrast in the gaps of the image degrades further because poorly resolved light from the hammerheads 420 spills into gaps 409. Contrast in the gaps 409 is similarly degraded if the mask rectangles are lengthened in an effort to compensate for line shortening in the printed patterns, because blurring is worse when the separating gaps are biased to be narrower.
Computer algorithms are known which can provide appropriate adjustment of mask shapes to compromise between such conflicting effects (see, for example, O. W. Otto et al., xe2x80x9cAutomated optical proximity correctionxe2x80x94a rules-based approach,xe2x80x9d in SPIE Proceedings, Vol. 2197xe2x80x94Optical Microlithography VII (Society of Photo-Optical Instrumentation Engineers, 1994), pages 278-293). However, these algorithms are only able to provide a very limited benefit when different aspects of image quality require that the shapes be perturbed in opposite directions, as with line shortening. In general, image enhancement techniques work poorly when geometric constraints that are inherent to the desired circuit pattern yield contradictory requirements for optimizing the shape and/or position of these patterns on the mask. For example, the close packing of patterns such as in FIG. 4 or FIG. 14 causes an intrinsic loss in contrast when mask features are biased to achieve the desired critical dimensions (CD""s) of the desired image.
Another class of enhancement techniques improves contrast in the image by shifting the phase of the light projected from the mask. This does not directly address the above-mentioned intrinsic geometric conflicts in certain circuit patterns, but it does reduce their severity by reducing image blur. One source of image blur is caused by the limited resolution of lithography lenses, which washes out the sharp transition in transmittance between mask features, blurring it over a distance defined by the lens resolution.
One enhancement technique (known as xe2x80x9cphase-shifting chromexe2x80x9d or xe2x80x9cattenuated phase shiftxe2x80x9d) improves image sharpness by augmenting the rate of change in illumination amplitude across the edge of mask features. This is achieved by using phase-shifting material of slightly negative transmittance for dark areas of the pattern, rather than the conventional material of zero transmittance, for example, as described in T. Terasawa et al., xe2x80x9cImaging characteristics of multi-phase-shifting and halftone phase-shifting masks,xe2x80x9d Japanese J. Appl. Phys. Part 1, Vol. 30, no.11B (1991), pages 2991-2997. Phase shifting increases the slope of illumination intensity at the edges of image features since the transmitted electric field makes a transition (see, for example electric field amplitude 160 in FIG. 1A) from unity to a value less than zero (see, for example electric field amplitude 160 in the dark region 199 in FIG. 1A); the slope in the image intensity across the edge of features in the image is increased accordingly. However, the steepness of the slope across the edges of image features is limited by the requirement that the negative electric field amplitude 160 transmitted to areas of the image that are intended to be dark areas 199 not have sufficient intensity 170 to print the dark areas 199 (FIG. 1A) as if they were bright areas 190. (For purposes of discussion, it is hereafter assumed that the photoresist is a positive resist, which is most commonly used in the art. In the case of a negative resist, dark image areas would be substituted for bright areas and vice versa.) Thus, while phase shifting improves contrast, the improvement can be inadequate in certain cases. As previously discussed, certain patterns are limited by intrinsic geometric constraints in which correction of dimensional errors can only be made at the expense of degraded contrast. Phase-shifting reduces the impact of these pattern conflicts, but does not eliminate them. The same conclusion applies when negative electric field amplitude is provided by a thin rim of transparent phase shifting material as discussed in A. Nitayama et al., xe2x80x9cNew phase-shifting mask with self-aligned phase shifters for quarter micron photolithography,xe2x80x9d in 1989 International Electron Devices Meetingxe2x80x94Technical Digest (Cat. 89CH2637-7) (Washington, D.C.: IEEE, 1989), pages 57-60.
The so-called alternating-phase-shift technique (for example, as discused in M. D. Levenson, N. S. Viswanathan, and R. A. Simpson, xe2x80x9cImproving Resolution in Photolithography with a Phase-Shifting Mask,xe2x80x9d IEEE Transactions on Electron Devices, Vol. ED-29, no. 12 (1982), pages 1828-1836) achieves further contrast improvement by successively shifting the phase of adjacent bright features between 0xc2x0 and 180xc2x0. In this way the contrast of illumination intensity across the edge of image features is further increased in comparison to either conventional masks or phase-shifting chrome. However, as with phase-shifting chrome, the alternating-phase-shift technique does not directly address the above-mentioned intrinsic geometric constraints of common 2D patterns, though it can further reduce their severity. In addition, with some 2D circuit layouts it is impossible to tile every mask feature with a phase that is opposite to the phase of all neighboring features, meaning that lithographic performance will be gated by the unimproved transitions that separate features having the same phase.
Moreover, the alternating-phase-shift technique often adds unwanted features to image patterns. This occurs when circuit shapes are laid out in such a way that the desired alternation in phase can only be achieved by introducing artificial 0xc2x0 to 180xc2x0 mask transitions which print as unwanted patterns. For example, when opposite phases are applied to bright regions that pass in close proximity to one another at a certain point on the mask, the phase must make such an unwanted transition if the bright regions are connected together elsewhere in the mask pattern. Such unwanted phase transitions will print as a dark fringe within the nominally bright connecting area, and must be trimmed away using a second exposure. It has also been suggested that unwanted mask transitions might be blunted below the threshold of printability through use of intermediate-phased regions, grading the transition in stages from 0xc2x0 to 180xc2x0 along the connecting regions. However, this gives rise to a phase tilt along the mask, which in turn causes very strong shifting of the image when focus fluctuates. For this reason intermediate phases are not often employed.
It is known that the benefits of a continuously varying phase can sometimes be achieved by tilting the light beam which illuminates the mask (for example, see N. Shiraishi et al., xe2x80x9cNew imaging technique for 64M DRAM,xe2x80x9d in SPIE Proceedings, Vol. 1674xe2x80x94Optical Microlithography V (Society of Photo-Optical Instrumentation Engineers, 1992), pages 741-752; M. Noguchi et al., xe2x80x9cSub-half-micron lithography system with phase-shifting effect,xe2x80x9d in SPIE Proceedings Vol.1674xe2x80x94Optical Microlithography V (Society of Photo-Optical Instrumentation Engineers, 1992), pages 92-104). With many patterns the tilt can be adjusted in such a way that the change in tilt phase along the mask causes the illumination at successive critical features to alternate between positive and negative phase. Moreover, where successive features are connected by orthogonal shapes, the phase makes a smooth transition from 0xc2x0 to 180xc2x0 along these connecting shapes. The above-mentioned focus sensitivity which such phase tilts can cause is avoided by illuminating the mask symmetrically from mirrored directions. In lowest order the focus sensitivities from the different directions then cancel.
Methods are known for selecting the illumination directions incident on a given mask in ways that maximize the slope of image features, and that minimize CD nonuniformity between different features through superposition of multiple illumination directions (for example, see U.S. Pat. No. 5,680,588 entitled xe2x80x9cMethod and system for optimizing illumination in an optical photolithography projection imaging systemxe2x80x9d issued to A. E. Rosenbluth and J. Gortych on Oct. 21, 1997). This is referred to as xe2x80x9csource optimizationxe2x80x9d. However, as with the image enhancement techniques described above, the benefit from optimizing the illumination in this way is limited. The optimized source achieves CD uniformity by balancing the differing bias effects of multiple illumination directions. Unfortunately, when bias effects are severe, for example when the geometric constraints of the pattern result in line shortening, such balancing usually requires contributions from image components produced by particular illumination directions that have low contrast.
Accordingly, there is a need for a technique for enhancing image quality that is not so strongly limited by the intrinsic geometric constraints of the pattern layout.
It is an object of the present invention to provide a method for optimally choosing illumination distribution and reticle mask features so that the number of adjustable degrees of freedom per resolution element is significantly increased.
It is a further object of the present invention to significantly reduce phenomena, such as line shortening, that are due to constraints inherent in the geometry of the desired wafer patterns.
It is a further object of the present invention to obtain optimal combinations of illumination and mask patterns without requiring that diffracted wave fronts collected by the lens be constrained to be symmetrical.
It is a further object of the present invention to obtain optimal mask patterns that are not constrained to conform to the basic layout of the desired target wafer patterns.
The present invention addresses the above-described objectives by providing a method for obtaining an optimal combination of source illumination and reticle mask features that are chosen such that resulting image is optimized in accordance with a user-specified merit function and constraints.
According to a first aspect of the invention, a merit function is chosen to describe a relationship between source parameters, reticle features or parameters, and desired image characteristics, and the merit function is optimized subject to user-specified constraints on the resulting image. Source parameters may include, for example, source direction and source intensities. Reticle parameters may be defined, for example, in terms of diffracted amplitudes. Image constraints may include, for example, a predefined intensity at desired image feature edge points, and thresholds defining bright and dark areas of the image. The merit function may include, for example, the gradient of the image perpendicular to the feature edges.
According to another aspect of the present invention, the features of a reticle using a desired mask material, such as phase-shifting chrome, are formed based on a set of optimized diffracted amplitudes in combination with an optimized set of source parameters.
According to another aspect of the present invention, the transmitting or diffracting features of a reticle are formed such that dark areas of the desired image pattern are produced by destructive interference of the diffracted energy. Such a mask may be formed, for example, using a phase-shifting material such as phase-shifting chrome.
Also, according to another aspect of the present invention, a lithographic system is provided that incorporates an optimized combination of source and reticle features, obtained using the method in accordance with the present invention, in order to print a desired pattern.
Also, according to a further aspect of the present invention, a computer program is provided that performs the method of obtaining a combination of source parameters and reticle features such that characteristics of a desired image are optimized in accordance with a merit function.
According to the present invention, a method is described for printing an integrated circuit pattern on a semiconductor wafer having a photoactive material thereon, the method comprising the steps of:
providing a desired wafer feature pattern having at least one wafer feature element;
deriving a merit function describing a relationship between an illumination source, a reticle, and an image, said source having at least one source parameter, said reticle having at least one diffractive feature, and said image having at least one image intensity;
selecting at least one constraint in relation to said desired wafer feature pattern that said at least one image intensity must satisfy;
selecting a combination of said at least one source parameter and said at least one diffractive feature so that said merit function is optimized in accordance with said at least one constraint;
illuminating said reticle with illumination energy from said illumination source, so that said illumination energy is diffracted by said reticle and projected through a lens aperture to form said at least one image intensity on the wafer;
exposing the photoactive material to said at least one image intensity; and
developing said exposed photoactive material to form a printed feature,
so that said printed feature conforms with said at least one wafer feature element of said desired wafer feature pattern in accordance with said constraints.
Also, according to the present invention, a method is described for selecting a combination of source illumination parameters and diffraction mask features for projecting energy through a lens aperture to form an image pattern on a wafer, the method comprising the steps of:
providing a desired wafer feature pattern having at least one wafer feature element;
deriving a merit function describing a relationship between an illumination source, a reticle, and the image pattern, said source having at least one source source parameter, said reticle having at least one diffractive feature, and said image pattern having at least one image intensity;
selecting at least one constraint in relation to said desired wafer feature pattern that said at least one image intensity must satisfy; and
selecting a combination of said at least one source parameter and said at least one diffractive feature,
so that said merit function is optimized in accordance with said at least one constraint.
A computer program product according to the present invention is described for selecting a combination of source illumination parameters and diffraction mask features for projecting energy through a lens aperture to form a desired image, the computer program product comprising computer readable instructions for performing a method having the following steps:
causing a computer to store a desired wafer pattern having at least one wafer feature element;
causing the computer to compute a merit function describing a relationship between an illumination source, a reticle, and an image pattern, said source having at least one source source parameter, said reticle having at least one diffractive feature, and said image pattern having at least one image intensity;
storing at least one constraint in relation to said desired wafer feature pattern that said at least one image intensity must satisfy; and
selecting a combination of said at least one source parameter and said at least one diffractive feature,
so that said merit function is optimized in accordance with said at least one constraint.
Also described, according to the present invention, is a machine readable storage medium having stored therein a program of instructions executable by the machine to perform method steps for selecting a combination of source illumination parameters and diffraction mask features for projecting energy through a lens aperture to form a desired image, said method steps comprising:
storing a desired wafer feature pattern having at least one wafer feature element;
storing instructions for causing a computer to compute a merit function describing a relationship between an illumination source, a reticle, and an image pattern, said source having at least one source source parameter, said reticle having at least one diffractive feature, and said image pattern having at least one image intensity;
storing at least one constraint in relation to said desired feature pattern that said at least one image intensity must satisfy; and
selecting a combination of said at least one source parameter and said at least one diffractive feature,
so that said merit function is optimized in accordance with said at least one constraint.
According to the present invention, a lithographic system is described for printing a desired wafer feature pattern on a semiconductor wafer including a photoactive material, the system comprising:
an illumination source having at least one source parameter;
a reticle having at least one diffractive feature; and
a lens;
said illumination source, said reticle and said lens being arranged so that said source illuminates said reticle so as to produce a plurality of diffracted amplitudes and said plurality of diffracted amplitudes are collected by said lens and projected to form a image on the semiconductor wafer, the image having at least one image intensity and wherein
said at least one source parameter and said at least one diffractive feature are selected in accordance with a merit function describing a relationship between said at least one source parameter, said plurality of diffracted amplitudes, and said at least one image intensity and wherein said merit function is optimized in accordance with at least one constraint that said at least one image intensity must satisfy,
so that exposing the photoactive material to said at least one image intensity and developing said exposed photoactive material forms at least one printed feature that substantially conforms with the desired wafer feature pattern.
A reticle according to the present invention is described for diffracting illumination energy to form a desired image pattern having a pattern of intensities, the desired image pattern having a bright area in which the intensities within the bright area exceed a predetermined bright threshold and having a dark area in which the intensities within the dark area are less than a predetermined dark threshold, the reticle comprising a pattern of phase-shifting material arranged so that the dark area is formed by destructive interference of diffracted illumination energy. A reticle according to the present invention is also described wherein said phase-shifting material comprises phase-shifting chrome material.
The novel features believed to be characteristic of this invention are set forth in the appended claims. The invention itself, however, as well as other objects and advantages thereof, may be best understood by reference to the following detailed description of an illustrated preferred embodiment to be read in conjunction with the accompanying drawings.